RFC: Mark "inlined by some callers" functions in BTF

[Song Liu <songliubraving@meta.com>]

The problem

Inlining can cause surprises to tracing users, especially when the tool
appears to be working. For example, with

    [root@ ~]# bpftrace -e 'kprobe:switch_mm {}'
    Attaching 1 probe...

The user may not realize switch_mm() is inlined by leave_mm(), and we are
not tracing the code path leave_mm => switch_mm. (This is x86_64, and both
functions are in arch/x86/mm/tlb.c.)

We have folks working on ideas to create offline tools to detect such
issues for critical use cases at compile time. However, I think it is
necessary to handle it at program load/attach time.


Detect "inlined by some callers" functions

This appears to be straightforward in pahole. Something like the following
should do the work:

diff --git i/btf_encoder.c w/btf_encoder.c
index fd040086827e..e546a059eb4b 100644
--- i/btf_encoder.c
+++ w/btf_encoder.c
@@ -885,6 +885,15 @@ static int32_t btf_encoder__add_func(struct btf_encoder *encoder, struct functio
        struct llvm_annotation *annot;
        const char *name;

+       if (function__inlined(fn)) {
+               /* This function is inlined by some callers. */
+       }
+
        btf_fnproto_id = btf_encoder__add_func_proto(encoder, &fn->proto);
        name = function__name(fn);
        btf_fn_id = btf_encoder__add_ref_type(encoder, BTF_KIND_FUNC, btf_fnproto_id, name, false);


Mark "inlined by some callers" functions

We have a few options to mark these functions.

1. We can set struct btf_type.info.kind_flag for inlined function. Or we
   can use a bit from info.vlen.

2. We can simply not generate btf for these functions. This is similar to
   --skip_encoding_btf_inconsistent_proto.


Handle tracing inlined functions

If we go with option 1 above, we have a few options to handle program
load/attach to "inlined by some callers" functions:

a) We can reject the load/attach;
b) We can rejuct the load/attach, unless the user set a new flag;
c) We can simply print a warning, and let the load/attach work.


Please share your comments on this. Is this something we want to handle?
If so, which of these options makes more sense?

Thanks,
Song